Hydrologic response of grasslands: Effects of grazing, interactive infiltration, and scale

Data collected at two measurement scales from a semiarid grassland are presented and analyzed to explore the hydrologic effects of grazing, interactions between overland flow and infiltration, and scale issues. Rainfall-runoff simulations were used to quantify the areal (3 by 10 m plot scale) response, and small-diameter (9 cm) disk infiltrometers were used to estimate point-scale hydraulic conductivity of bare and vegetated soil. Plot-scale data show that grazing increases runoff overall, a common result, but infiltrometer measurements indicate that only the point-scale hydraulic conductivity of vegetated soil is changed by grazing. In light of this and the well-known relationship between microtopography and vegetation in semiarid grasslands, we hypothesize that small-scale surface interactions (in particular, the so-called run-on phenomenon) are a significant component of the observed effects of grazing, as well as a factor in the hydrologic response of grasslands. Results obtained from high-resolution numerical simulations support this hypothesis. This phenomenon is not captured by classical infiltration theory or by the usual methods of statistical parameterization. In general, interactions are more likely to be important as spatial variability increases, and the relative importance of these interactions will be a function of the spatial structure of the variability and the hydrodynamics of overland flow, as controlled by microtopography.